Burst wave lithotripsy (BWL) and ultrasonic propulsion are promising ultrasound technologies being developed as rapidly as possible with funding from NIH NIDDK Program Project grant DK043881 and NASA. The goal is to create an instrument, similar to an ultrasound imager, with a hand-held probe to image, break and reposition stones in awake subjects in the office or clinic to facilitate clearance of the stones and fragments. This is an update on status.

Over 30 participants have been treated with BWL in a number of trials at various sites despite the necessary interruptions caused by the pandemic. Those studies are ongoing; however, the results of the very first 2 cases of BWL have been recently published.¹ The first participant was treated during ureteroscopy under anesthesia as part of a trial designed to directly observe fragmentation and any papillary injury. The stone was fractured to fragments under 2 mm in 9 minutes. Fragmentation was observed visibly and with real-time ultrasound monitoring, and ultrasonic propulsion was used to clear some of the debris from the calyx. Mild hematuria was observed. The second participant had a 7 mm ureterovesical junction (UVJ) stone and was treated awake without anesthesia in the clinic. He tolerated the procedure without pain from BWL and later passed the stone. We look to complete these trials in 2021, and are working on a new transducer design to expand the beam width to fragment larger stones (fig. 1).¹

Figure 1. Acoustic pressure field map of a broader (15 mm) beam, that accelerated fragmentation of 15 mm stones by a factor of 3 in vitro. (Figure courtesy of Akshay Randad M.S., APL-UW)

Ultrasonic propulsion alone has been used safely in over 70 human volunteers to reposition small stones. In our most recent study, a 3 mm stone was pushed from the UVJ into the bladder and the patient felt immediate relief. Ultrasonic propulsion has been used in vitro to disperse fragments to assess if a stone has been comminuted by BWL.² Lastly both BWL and ultrasonic propulsion can enhance the twinkling artifact making stones and fragments more conspicuous for locating and targeting during treatment.³

The research group continues novel stone research particularly focused around these technologies and ultrasound imaging. Suppression of twinkling in stone formers in a hyperbaric chamber provides evidence bubbles contribute to the Color Doppler twinkling artifact in humans.⁴ Cavitation from these bubbles naturally present in the human body is minimized in BWL to allow the stress and strain within the stone to grow fractures. A recent in vitro study of BWL showed how the complex wave pattern created by the BWL pulses causes a cracking pattern throughout the stone and fragments proportional to the BWL wavelength (fig. 2).⁵ Ultrasonic propulsion is also being more fully understood and improved. A recent ultrasonic propulsion study used transcutaneous ultrasound to trap, lift, and move a model stone through a complex 3-D pattern inside a porcine bladder, a technology we are calling “acoustic forceps. ” ⁶ We are pursuing efforts to expand research of these technologies to treat pediatric stone formers and fragile populations such as individuals with spinal cord injury as we feel they could potentially be quite valuable.

Figure 2. Photoelastic image of the stress pattern created in cylindrical model stone by a BWL pulse initially propagating to the right. (Figure courtesy of Adam Maxwell Ph.D., APL-UW)

We have completed our work with NASA. Stones have long been a top risk of long duration space flight as over 30 stone events have occurred within 2 years of space flight and one cosmonaut reported passing a stone in space. As such, NASA has helped fund our research, and our technology has now been implemented into the NASA 1 Flexible Ultrasound Unit. The results of our research have contributed to a recommendation from NASA leadership to its Human System Risk Board to reduce the risk level of the renal stone risk in space flight.

Both BWL and ultrasonic propulsion were exclusively licensed from the University of Washington to SonoMotion, Inc., for commercialization. SonoMotion is working towards regulatory approval of a single platform that will provide ultrasonic propulsion and BWL capabilities, which will be called Stone Clear and Break Wave, respectively.

We appreciate all the support we have received from the urology community and continue to work to get these innovative technologies into the hands of practicing urologists.